This invention relates to charge storage structures of the kind known as "electrets," a neologism which designates an insulating material containing permanently stored electrical charges (retention time of the order of several years).
Electrets are generally made in the form of a thin membrane (a few tens of microns) of a synthetic organic polymer, such as polypropylene, polychlorofluorethylene, polyvinyl carbazole, etc. A flexible membrane such as this arranged between two electrodes has often been used for forming, by virtue of the electrical field developed by the stored charges, a transducer which converts the acoustic vibrations of the ambient air into electrical vibrations (microphone) and electrical vibrations into acoustic vibrations (receiver).
The quantity of charges in an insulating material is limited by the nature of the dielectric constituting the membrane. Accordingly, the electrical field is often far from that which the dielectric used for forming the electret could tolerate without disruption. In the case of a microphone, a preamplifier is generally necessary in spite of the gain of the electret transducer in relation to the conventional capacitor microphone.
The invention enables all or some of the above limitations to be overcome.